Human flavor perception is complicated, involving a variety of external sensory stimuli and internal states. Not only does it involve the five classical senses, but also sensing through the gut, and the emotional, memory-related, motivational, and linguistic aspects of food. First, there are the basic tastes: sweet, sour, salty, bitter, and umami. The smell of foods is the key contributor to flavor perception, which is in turn a property of the chemical compounds contained in the ingredients. There are typically tens to hundreds of different flavor compounds per food ingredient.
In addition, many children, as well as adults, are picky eaters. Most children eat a wide variety of foods until they are around two years old, when they suddenly stop. The phase can last until a child is four or five years of age. It is believed to be an evolutionary response. Toddlers' taste buds shut down at about the time they start walking, giving them more control over what they eat. However, junk food such as ice cream, French fries and soda are often more attractive to the eater than healthy foods such as brown rice and broccoli. This is also believed to be an evolutionary instinct useful when humans used to wander around in the woods searching for food. In the distant past, humans depended heavily on their senses to make a decision of what to eat and what not to eat. In nature, foods that are sweet are almost always safe to eat and are nutritious. They make hunger go away; foods that smell odd, or taste bitter or sour usually mean they are potentially toxic or spoiled, and less safe to eat. In the modern environment, where food is bought in supermarkets or restaurants, those same survival instincts often serve instead to make humans obese and chronically ill.